Kinetic studies on ATP- and Na- dependent ion transport mechanisms were carried out. The intermediate steps in the overall catalytic reaction mediated by the Ca-ATPase, and Na+K ATPase enzymes were examined with respect to the dephoshorylation of the phosphoenzyme by ADP. The phosphoenzymes in the sarcoplasmic reticulum from hearts of aged rats turn over more slowly than did the enzyme from young animals. The rapid kinetics of the Ca/Na exchange reaction, which mediates the uptake of Ca across the sarcolemma, was described. The mechanism of ATP synthesis in subchloroplast particles was clarified.